1. Field of the Invention
The invention relates to a manufacturing method of a photonic band gap fiber and the photonic band gap fiber.
2. Description of the Related Art
A Holey fiber (HF) or a photonic crystal fiber (PCF) is a new type optical fiber that realizes an optical transmission using a principle of total internal reflection by lowering an average refractive index of a cladding portion by regularly disposing holes on the cladding portion around a core portion. It is expected that the HF makes a core diameter of the core portion large thereby to increase an effective core area (Aeff) so that the HF is applied as a low optical nonlinear transmission medium for an optical telecommunication and a fiber laser. In particular, when an ultra large Aeff whose characteristics are difficult to control through a conventional solid type fiber is intended to be realized, it is effective to control the characteristics by employing a structure of the HF. For example, M. D. Nielsen, et al., Electron. Letters, Vol. 39, No. 25, PP1802-1803, 2003 (hereinafter to be referred to as Nielsen) discloses a technology for increasing the Aeff to 600 mm2 using the HF.
However, in the HF, when it is intended to realize a low optical nonlinearity by increasing the Aeff, a problem arises in that bending loss characteristics, which are one of important factors of optical fiber characteristics, are outstandingly deteriorated. For example, Nielsen describes that a transmission loss of the HF increases in a wavelength of 1550 nm even if the HF is wound around a bobbin with a relatively large bending diameter of 80 mm. Further, the HF has characteristics that a bending loss increases when the HF is used in a wavelength band of 1.0 μm or less, to which attention is paid regarding the use of the HF for the fiber laser and the like.
In contrast, a photonic band-gap fiber (PBGF) is an optical fiber of a type that realizes an optical transmission by disposing holes on a cladding portion so as to form a photonic crystal, thereby forming a photonic band-gap by a two-dimensional Bragg reflection in a wavelength of light to be transmitted, and introducing a core portion to the photonic crystal as a crystal defect. For example, K. Saitoh, et al., OPTICS EXPRESS, Vol. 11, No. 23, 2003, pp 3100-3109 (hereinafter to be referred to as Saitoh) discloses an air-core type photonic band gap fiber that can realize a low bending loss by very strong light-confinement while realizing an ultra low nonlinearity by increasing an Aeff. Further, Saitoh discloses a calculation method of optimally designing a profile parameter of a photonic band-gap fiber in detail.